The present invention relates generally to improvements in television receiver tuner circuits and, in particular, to impedance matching apparatus useful for improving the operation of a multiband, voltage-controlled television receiver tuning system.
A television receiver comprising a single conversion superheterodyne tuning system typically includes a tunable RF amplifier connected for coupling received VHF and CATV television signals through one or more filter circuits to a mixer. The mixer also receives a local oscillator signal which is heterodyned with the received radio frequency VHF and CATV television signals to develop an intermediate frequency signal for subsequent processing by the television receiver. Received radio frequency UHF television signals are typically converted to intermediate frequency signals in a separate section of the tuner and coupled through the VHF mixer, which is operated as an amplifier in response to the intermediate frequency UHF signals, for subsequent processing by the television receiver.
In order to tune the various television signal frequency bands, as well as the individual television channels within each band, each stage of the tuner comprises a tunable resonant circuit which is responsive to bandswitching signals for facilitating tuning of each of the frequency bands and includes a variable reactance device, normally a varactor diode, responsive to a DC tuning voltage for tuning to different channels within a selected frequency band. One of these tunable resonant circuits is normally connected to an input of the VHF mixer stage which typically comprises a field effect transistor (FET) so used because of its relatively low noise figure and excellent cross modulation characteristics.
U.S. Pat. No. 4,048,598 teaches that FET's exhibit a rather large input parasitic capacitance which is on the same order of magnitude as the desired capacitance of varactor diodes used in the UHF range. In addition, this patent teaches that the impedance of a varactor tuned resonant circuit selected for use in the UHF band is relatively low at the low end of the UHF range while a FET is a relatively high impedance device. As a result of this impedance mismatch, the operation of the circuit is relatively poor at the low end of the UHF band although, since the impedance of the varactor tuned circuit increase with frequency, satisfactory operation is achieved at the high end of the UHF range. To overcome these problems, the foregoing patent discloses the use of impedance transformation means connected to the gate electrode of the FET; the impedance transformation means comprising a series capacitor and shunt inductor whose values are selected to provide a resonant point at a frequency below the lowest frequency in the UHF band. The impedance transformation means provides improved circuit performance at the low end of the UHF range while providing relatively little degradation at the high end of the range. For a similar purpose, the use of an impedance matching inductor in association with a VHF mixer stage is known for improving the performance of a tuner operating over the superband range of television signal frequencies.
In each of the foregoing prior art examples, a single inductor having a certain selected value of inductance is connected in shunt across the gate electrode of an FET to match the impedance of the FET to the impedance of a varactor tuned resonant circuit for the low end of a single band of television signal frequencies. Although this technique is adequate for improving the operating performance of a tuner when considering only a single band of television signal frequencies, its use cannot be extended to improve the performance of a multiband tuner where different impedance matching requirements must be satisfied for each different tunable band. In particular, while the selected value of inductance of the inductor may be appropriate for providing a good impedance match for one band, this value will generally not provide an acceptable impedance match in other bands.
Accordingly, it is a primary object of the present invention to provide a novel tuner circuit for a television receiver which is capable of improved performance over a plurality of television signal frequency bands.
It is a more specific object of the invention to provide a circuit adapted for matching the impedance of a varactor tuned resonant circuit to the input impedance of an active device such as a field effect transistor at the low end of each of a plurality of television signal frequency bands.